Which Numidian king's defection helped Rome win Zama?

Masinissa, whose Numidian cavalry had been Carthage's best arm for years and rode for Rome at Zama in 202 BC. Scipio had spent the African campaign backing him against a rival claimant, which is the whole strategy in miniature: Rome did not out-fight Hannibal so much as take his horsemen away.

The battle turned on the elephants failing. Scipio arranged his lines with lanes down the middle instead of a solid front, so the charge ran through rather than into him, and had trumpets blown to panic the animals back through their own side. With the elephants spent and the flanks stripped of cavalry, Masinissa and Laelius drove Carthage's remaining horse off the field, came back behind the infantry, and ended it.

Masinissa then reigned for fifty years and spent them provoking his neighbour. Carthage's treaty forbade war without Roman permission, so he raided and annexed, and when Carthage finally defended itself Rome had the pretext it wanted for the Third Punic War and the destruction of the city. He was still leading cavalry in his eighties, reportedly fathered a son at 86, and outlived Hannibal, who was hunted across the eastern Mediterranean and took poison rather than be handed over.